1. Field of the Invention
This invention relates to a process for efficiently removing and recovering carbon dioxide (CO.sub.2) from combustion exhaust gases leaving the boilers of thermal power plants.
2. Description of the Related Art
In recent years the greenhouse effect of CO.sub.2 has arrested attention as a factor contributing to the global warming. Counteracting this effect is urgently needed throughout the world so as to protect the global environment. The source of CO.sub.2 is omnipresent in every area of human activities that involve combustion of fossil fuels, and the tendency is toward stricter emission control than before. In view of these, factors many studies are under way on the recovery of CO.sub.2 from combustion exhaust gases, especially from those emitted by power-generating installations such as steam power plants that burn huge volumes of fossil fuels, and on the storage of the recovered CO.sub.2 without releasing it to the atmosphere.
The present applicant previously proposed a process for the removal and recovery of CO.sub.2 from combustion exhaust gases with less energy consumption, as illustrated in FIG. 3 (Japanese Patent Provisional Publication (Kokai) No. 3-193116).
In FIG. 3, CO.sub.2 -containing combustion exhaust gas from a boiler 1 is boosted to a high pressure by a boiler combustion gas fan 14 and delivered to a combustion gas cooler 15, where it is cooled with cooling water 16 and transferred to a CO.sub.2 -absorption column 18, while the spent cooling water 17 is discharged out of the system.
Inside the CO.sub.2 -absorption column 18, the combustion exhaust gas comes in countercurrent contact with regenerated CO.sub.2 -absorbing liquid 19 containing an alkanolamine, and through a chemical reaction the CO.sub.2 in the gas is absorbed by the liquid. The gas 21 freed of CO.sub.2 is discharged from the system. The absorbing liquid 20 that has absorbed CO.sub.2 is sent, after pressure boost by a rich solvent pump 22, to a rich/lean solvent heat exchanger 23, where it is heated by the regenerated absorbing liquid and then supplied to a CO.sub.2 -absorbing liquid regeneration column 24.
At a lower portion of the regeneration column 24, the CO.sub.2 -absorbing liquid is heated in a reboiler 30 by low pressure steam (at an absolute pressure of 4 kg/cm.sup.2 G) 13 extracted from a low pressure turbine 8. CO.sub.2 gas entraining steam is conducted from the top of the CO.sub.2 -absorbing liquid regeneration column 24 to an overhead condenser 25. A condensate of low pressure steam, condensed by the reboiler 30, is boosted by a reboiler condensing pump 32, mixed with preheated boiler feed water to raise the temperature of the feed water, and the mixture is fed to the boiler 1.
The CO.sub.2 discharged, entraining by steam, from the CO.sub.2 -absorbing liquid regeneration column 24 preheats in the overhead condenser 25 the boiler feed water whose pressure has been boosted by the boiler feed water pump 12. The steam-entraining CO.sub.2 is then cooled by an overhead cooler 26 and separated from water by a separator 27, and CO.sub.2 alone is led through line 28 to another process step for recovery. The water separated by the separator 27 is pumped back to the CO.sub.2 -absorbing liquid regeneration column 24 by a condensing water circulating pump 29.
The regenerated CO.sub.2 -absorbing liquid is boosted to a high pressure by a lean solvent pump 31, cooled in the rich/lean solvent heat exchanger 23 with the absorbing liquid that has absorbed CO.sub.2, cooled further by a lean solvent cooler 33, and then supplied to the CO.sub.2 -absorption column 18.
In the meantime steam 2 at a high pressure and high temperature that has been generated and heated by the boiler 1 is caused to drive a high pressure steam turbine 3, heated by a reheater 5 in the boiler 1 as an emission 4 from the turbine, and delivered as reheated intermediate pressure steam 6 to the low pressure turbine 8.
Part of the steam is extracted at line 13 from the low pressure section of the low pressure turbine 8 and supplied to the reboiler 30. The rest of the steam 9 exhausted from the low pressure turbine is condensed by a condenser 10, and the condensate 11 is led to the overhead condenser 25 by the boiler feed water pump 12.
Examples of alkanolamines that absorb CO.sub.2 include monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, and diglycolamine. An aqueous solution of such a single alkanolamine or of two or more such alkanolamines is used. Usually, an aqueous monoethanolamine solution is preferred.
The above-described process reduces the power generation efficiency of a power plant compared with a plant that does not adopt the process for CO.sub.2 removal, but the degree of efficiency drop can be kept low. For example, when 90% of CO.sub.2 in the combustion exhaust gas from the boiler of a natural gas-fired power plant is to be removed, if the supply of heat for heating the reboiler 30 is obtained by combustion of fuel, the required fuel would amount to 18.9% of the heat of combustion in the boiler of the power plant. Consequently, the power generation efficiency for the same quantity of heat of combustion would decrease by 6.3%, from 36.4% for non-CO.sub.2 removal operation to 30.1% with CO.sub.2 removal. According to the process proposed as above, however, steam at a pressure of 4 kg/cm.sup.2 G is extracted from the low pressure steam turbine 8 to heat the reboiler 30, and the condensate of the steam can heat boiler feed water. Moreover, the heat exchange in the overhead condenser 25 between the steam-entraining CO.sub.2 from the CO.sub.2 -absorbing liquid regeneration column and the boiler feed water renders it possible to decrease the quantity of steam extraction otherwise required to heat the boiler feed water. Thus, while the axial power of the low pressure steam turbine decreased to some extent, a drop in the power generation efficiency for the same quantity of heat of combustion could be limited to 4.5%, attaining a 1.8% improvement in the power generation efficiency over the conventional process. Also, when a combined cycle gas turbine is adopted, an improvement of 3.4% was shown to be achieved.
Although the above-described proposed process can limit the deterioration of the power generation efficiency owing to the removal and recovery of CO.sub.2 to some extent, there is strong demand for more improvements which would lessen the penalty of efficiency drop further.